612 ZOOLOGY sect. 



mass, except at one point where a small area, the blastopore, 

 is uncovered. The central mass is the endoderm; the lumen 

 of the enteron is formed by coalescence of the vacuoles. 



In accordance with the smaller size of the ova and the rela- 

 tionship of the embryo with the wall of the uterus, the American 

 species show a totally different mode of development. The eggs, 

 which are almost entirely devoid of yolk, undergo a total and 

 tolerably equal process of segmentation. Even at this stage the 

 embryo, which increases considerably in size, appears to receive 

 nutrient lymjih from the uterine wall. When it has reached the 

 32-cell stage the embryo, according to one observer, consists of a 

 solid mass closely invested by the epithelium of the wall of the 

 uterus. It then becomes reduced in size, and owing to exosmosis, 

 assumes the form of a disk placed in close apposition to one side 

 of the wall of the uterus. The embryo subsequently loses its 

 flattened form and becomes somewhat vesicular, the cavity of the 

 vesicle opening into the cavity of the uterus. From its surface 

 are given off isolated cells which become applied in part to the 

 wall of the uterus, and finally unite to form a complete envelope 

 {amnion) enclosing the embryo. The vesicle then becomes closed 

 and the embryo raised from the surface of the uterine wall, 

 the part applied to the latter narrowing so as to form a sort 

 of stalk, at the base of which is a growth of cells termed the 

 placenta. Into close relation with this placenta comes a ring- 

 shaped thickening of the uterine wall, the %i,terine placenta. 



In P. aqjensis (Fig. 485) proliferation of cells gives rise 

 to an oval thickening behind the elongated blastopore. The 

 mesoderm takes its origin at this point and extends forwards 

 in the form of two germinal lands, one on the right of the 

 blastopore and the other on the left. These bauds undergo a 

 division into rudiments of segments — the division beginning in 

 front. The lips of the blastopore meanwhile become approximated, 

 and fuse throughout the greater part of their length, leaving only 

 an anterior and a posterior opening ; these go to form the mouth 

 and the anus respectively. The division into segments soon 

 becomes well marked. The cavities of the segments give rise 

 only to the nephridia and the generative ducts, which thus alone 

 rej)resent the coelome. At the anterior end the head lobes 

 become distinguishable. The hoAj elongates, and the head and 

 trunk become differentiated. The limbs now arise as ventro- 

 lateral outgrowths which are developed from before backwards. 



Distribution. — The various species of Peripatus are all terres- 

 trial, and are found in damp localities, under bark, or dead timber, 

 or stones. Some twenty-nine species occur in the Neotropical 

 region ; one in South America ; eight in Africa ; four in Malaya ; 

 one in New Britain, and eight in Australasia. 



Relationships. — Peripatus is the most primitive of existing 



